1. Field of the invention
This invention relates to a paper feeding device for use in a printer.
2. Description of the prior art
Japanese Kokai No. 60-72773 discloses a thermal color printer in which an image is formed on a recording paper using a multi-colored thermal transfer ribbon by heat generated by a thermal head. The recording paper is repeatedly fed through a transfer or printing station by the forward and reverse rotation of a platen roller with the ribbon being advanced to present its different colors, thus performing multi-color printing.
A cross-sectional view of this prior art printer is shown in FIG. 1, wherein paper 3 is stacked in a cassette 2 which is removably mounted on a body 1 and is fed sheet by sheet by a feed roller 4. A thermal transfer ribbon 7 is wound on a supply reel 5 and a take up reel 6. A thermal head 8 is disposed facing a platen roller 9 and is moved up and down by an electromagnet 12. A pinch roller 11 on the paper feed side and a pinch roller 10 on the paper discharge side are pressed against the platen roller by springs (not shown). The pinch rollers are frictionally driven by the rotation of the platen roller. A friction member 13 and a brake 14 are provided above the pinch roller 11 on the feed side, together with aligning rollers 15, 16. Paper discharge guides 24, 25 are disposed between the pinch roller 10 on the discharge side and a pair of paper discharge rollers 17, 18, and a paper front or leading edge detector/sensor 19 is disposed at the lower end of the discharge guides. Paper guides 20, 21, 22 are provided above the pair of aligning rollers 15, 16 to guide the sheets of paper from the cassette 2 and from a manual feed tray 25. The printer body includes a stacker 26, a power source 27 and a control panel 28.
In operation, a sheet of paper taken out of the cassette 2 by the rotation of the feed roller 4 passes between the paper guides 20, 21, and is fed by the aligning rollers 15, 16 into the nip between the platen roller 9 and the pinch roller 11 on the paper feed side. The paper is then advanced between the platen roller and the transfer ribbon 7 and through the nip at the pinch roller 10 on the discharge side until its leading edge reaches the sensor 19. At this point appropriate electrical signals are applied to the thermal head 8, and the printing is started for a first color. The printed paper advances through the discharge guides 24, 25 and between the rollers 17, 18 until its trailing edge arrives at the nip between the platen roller 9 and the pinch roller 11 on the feed side. The paper is then returned by the reverse rotation of the platen roller until its leading edge arrives at the sensor 19. The same printing operation is then repeated for each color on the ribbon 7, and the sheet(s) is finally discharged to the stacker 26.
With such a prior art thermal printer the paper sometimes becomes loosened and disengaged from the surface of the platen roller due to its repeated conveyance in the forward and reverse directions, and the pure "idling" operation of the pinch rollers 10, 11. This results in misalignments or offsets between the different color prints or separations, and produces undesirable fringe patterns.